linux-kernel - Re: [V2 PATCH 4/8] KVM: selftests: x86: Precompute the result for is_{intel,amd}

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Message-ID: <YyuEhxW4URWVvHyW@google.com>
Date:   Wed, 21 Sep 2022 14:39:19 -0700
From:   David Matlack <dmatlack@...gle.com>
To:     Vishal Annapurve <vannapurve@...gle.com>
Cc:     x86@...nel.org, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org, pbonzini@...hat.com,
        shuah@...nel.org, bgardon@...gle.com, seanjc@...gle.com,
        oupton@...gle.com, peterx@...hat.com, vkuznets@...hat.com
Subject: Re: [V2 PATCH 4/8] KVM: selftests: x86: Precompute the result for
 is_{intel,amd}_cpu()

On Thu, Sep 15, 2022 at 12:04:44AM +0000, Vishal Annapurve wrote:
> Cache the vendor CPU type in a global variable so that multiple calls
> to is_intel_cpu() do not need to re-execute CPUID.
> 
> Add cpu vendor check in kvm_hypercall() so that it executes correct
> vmcall/vmmcall instruction when running on Intel/AMD hosts. This avoids
> exit to KVM which anyway tries to patch the instruction according to
> the cpu type.

Out of curiousity, why do we want to avoid this exit?